This invention relates to an etching method of diamond which will be applicable to fabrication of diamond devices, e.g. diodes, transistors or integrated circuits which make use of diamond as active semiconductor layers. Diamond attracts attention, because diamond has a possibility of making devices which can stably work under rigorous conditions, e.g. at high temperature or under the exposure of radioactive rays, or devices which can generate high output power.
Since diamond has a wide band gap of 5.5 eV, the temperature range in which diamond acts as an intrinsic semiconductor does not exist below 1400.degree. C., at which diamond is still thermally stable. Accordingly, diamond devices should stably work at high temperature. Intrinsic semiconductor is defined as a semiconductor having the same number of electrons and holes. Thus, when temperature rises above the intrinsic region of a semiconductor, the semiconductor device would lose its function as an active device, e.g. amplification, rectification, oscillation, etc. Of course diamond is also chemically stable. The heat conductivity of diamond is 20 W/cmK, which is more than ten times larger than that of silicon (Si). Thus diamond also excels in heat diffusivity.
Diamond is favored with high carrier mobilities; electron mobility is 2000 cm.sup.2 /Vsec and hole mobility is 2100 cm.sup.2 /Vsec at 300 K. Furthermore, the dielectric constant of diamond is small (.epsilon.=5.5). These advantages would make diamond a suitable material for manufacturing high speed devices or high frequency devices. In addition, diamond has a large breakdown voltage (E=5.times.10.sup.6 V/cm). Due to the advantage, diamond is deemed as a promising material for manufacturing high frequency devices with high output power.
Manufacturing these devices including diamond layers requires such a technique for forming various kinds of films on a substrate. In addition to the film forming technique, an etching technique, e.g. forming steps or grooves on a diamond film or removing unnecessary portions of a diamond film is indispensable.
Various etching techniques have been established for silicon (Si) semiconductor. The most pertinent technique should be chosen according to the purpose of etching. Diamond has chemical and physical properties different from silicon. Diamond is much stronger chemically or physically than silicon. Therefore, the etching technique of silicon is not applicable to the etching of diamond as it is.
N. N. Efremow had reported an etching method of diamond making use of ion beams. N. N. Efremow et al: J.Vac. Sci. Technol. B3(1), 1985, p416
However, this method was not a practical method for etching a diamond semiconductor with wide area. Reactive ion etching method (RIE) has been used for etching silicon. The RIE method is also applicable for wide silicon substrates. The RIE method is now inapplicable to diamond. However, this invention wishes to apply the RIE method to the diamond etching. A gas including oxygen was deemed to be a pertinent gas for etching diamond. However, the reactive etching method of diamond using oxygen plasma makes many random concave parts and convex parts on the surface of diamond. The roughness of the etched surface of the diamond makes it impossible to deposit various films on the surface. Thus, the lack of an effective etching method has hindered us from manufacturing diamond semiconductor devices. The reason why the usual RIE method generates a rough surface on diamond has not been clearly understood yet. One of the probable reasons is that because of the high temperature of the oxygen plasma, the kinetic energy of oxygen ions is so high that the oxygen ions would bombard the diamond crystal with strong impulses. The impulse would produce the roughness of surface. Another possibility is that oxygen ions are moving randomly with various energies and momenta in heated plasma and the impulses of the bombardment are so random that the etching effect would be ununiform.
If ununiform impulses of oxygen ions would induce the roughness of diamond surface, the bombardment of oxygen ions with a common energy and with a common velocity would enable us to etch a diamond surface with high flatness.
However, the reactive ion etching method excites a reactive gas into plasma by applying high frequency electric field between two electrodes in a vacuum chamber. Therefore, there is no means for controlling the energy or velocity of oxygen ions. Oxygen ions gain or lose the momentum or energy by collision with other oxygen ions or electrons. The plasma is in general not kept in thermal equilibrium. The energy of ions is diverse. The average kinetic energy of electrons is higher than the average kinetic energy of ions in spite of frequent collisions between ions and electrons. Thus, the conventional reactive ion etching (RIE) method lacks the means of controlling the energies or momenta of ions in plasma. The strength of electric field would affect the production rate of plasma. However, the energies or momenta of ions cannot be controlled by the strength of electric field.
The purpose of this invention is to provide an etching method of diamond which can etch a diamond surface without concave parts or convex parts in order to fabricate diamond semiconductor devices with high performance.